Gadenin日本語Get the appAutumn sowing beats spring sowing, and it is not close
A sweet pea sown in autumn spends the winter building root rather than shoot. By the time growth restarts in late winter it has a root system several times the size of anything a spring sowing can put together, and that root is what carries the plant through the heat that eventually ends its season. Autumn sown plants flower two to four weeks earlier, on longer stems, and keep going noticeably later into the summer than plants sown after the turn of the year.
Sow in mid autumn, once the worst summer heat has gone but with six to eight weeks of mild weather still ahead. The aim is a seedling with three or four leaves and a full pot of root before hard cold arrives. Sown too late, the seed sits in cold wet compost and rots. Sown too early, the plant becomes a leggy tangle that has to be cut back hard anyway.
Overwinter the pots in a cold frame, an unheated greenhouse, or against a sheltered wall with the pots grouped together. Sweet peas are hardy and want cold, not warmth. A heated windowsill produces pale drawn stems that flop the moment they go outside. The only real winter enemies are waterlogged compost and mice, and both are easier to deal with under a cold frame lid than in the open.
Late winter sowing is the fallback, not a failure. Where winters are severe enough that a cold frame freezes solid for weeks, or where autumn sown plants simply drown, sow in late winter under cover instead and accept a display that starts later. The technique is identical; only the calendar moves.
| Sowing time | What you get | Suits |
|---|---|---|
| Mid autumn, overwintered cold | Strongest plants, earliest and longest flowering | Mild to moderate winters with a cold frame or sheltered wall |
| Late winter under cover | Two to four weeks later, slightly shorter stems | Hard winters, or where autumn sowings rot |
| Early spring under cover | A useful display, shorter season before summer heat | A late start, or a second batch to follow the first |
| Direct sown outdoors in spring | Patchy stand, later flowering, mice take some seed | Mild springs and free draining ground only |
Depth, containers and the deep root run
Sow one inch (2.5 cm) deep. That is deeper than most flower seed and it matters: a sweet pea seed at half that depth dries out in the first warm spell and the emerging shoot is easily knocked over. Use one seed per cell or two or three to a pot, and firm the compost lightly rather than packing it down.
The container needs depth far more than width. Sweet peas send a root straight down and resent having it turned back on itself at the bottom of a shallow pot. Root trainers, deep cells, four inch (10 cm) pots at least five inches (12 cm) deep, or the cardboard tubes from a roll of kitchen paper all work. A seed tray does not; the roots circle within weeks and the plants never recover their vigor.
Use ordinary all-purpose potting mix with some added grit or perlite. Sweet peas want moisture but they will not tolerate a compost that stays soaking through a cold winter, and most of the losses blamed on frost are actually rot at the base of a waterlogged pot. Water thoroughly after sowing and then only when the surface has dried, which in winter can mean once every ten days.
Soaking or chipping the seed is optional and often counterproductive. Fresh seed with a normal coat takes up water perfectly well on its own and germinates in ten to fourteen days at 55 to 65F (13 to 18C). Only the very hardest, smoothest seeds benefit, and a seed soaked until it is soft and swollen rots readily in cold compost. If you soak at all, do it for a few hours, not overnight, and sow the same day.
- Sow one inch deep into a deep container Fill root trainers or deep pots with gritty all-purpose potting mix, push each seed down to a full inch (2.5 cm), cover and firm gently. Water once, thoroughly, and then leave the compost to dry back at the surface before watering again.
- Keep them cold, bright and airy A cold frame, unheated greenhouse or sheltered wall is right. Open the frame on mild days so air moves over the seedlings. Warmth is the enemy: seedlings raised indoors come out pale, drawn and unable to stand up on their own.
- Protect the seed from mice Mice dig up sweet pea seed and can empty a whole tray in one night, leaving neat holes and no seedlings. Cover the pots with mesh or a sheet of glass until the shoots are up, or set traps in the frame.
- Pinch the tip at four leaves When a seedling has three or four true leaves, pinch the growing tip out just above a leaf joint. This forces side shoots from the base and turns one thin stem into three or four strong ones. Skipping this gives a single spindly vine that flowers thinly.
Why pinching at four leaves changes the whole plant
An unpinched sweet pea grows as one leader. It climbs quickly, looks promising, and then flowers only at the top on a handful of stems. Pinching the tip out at three or four leaves breaks that dominance, and the plant answers by pushing shoots from the base. Those basal shoots are thicker than the original stem, they flower lower down where you can reach and smell them, and there are three or four of them instead of one.
Do it on the seedling, not on a plant that is already a foot tall. The earlier the pinch, the more strongly the base responds. On autumn sown plants, pinch in autumn once the fourth leaf is open, and expect the side shoots to appear slowly through winter and then race away in late winter. If a plant grew leggy before you got to it, cut it back to two or three leaf joints in late winter and it will still break from the base.
The pinched tip is not worth keeping. It will not root and it carries no flower buds. Take it off cleanly with fingers or snips just above a joint, leaving no stub to die back.
Record the sowing date, the emergence date and the date you pinched each batch. Two seasons of that record tells you the latest autumn sowing that still overwinters well in your own garden, and whether the extra weeks of an early autumn sowing actually paid for themselves in earlier flowers.
Sweet pea seeds and pods are poisonous. They look like garden peas and are produced by a plant in the same family, but they are not edible and eating them, particularly in quantity, causes serious illness. Keep the packet, soaked seed and collected pods away from children and pets, and never store them with vegetable seed.
Hardening off and planting out
Plants raised in a frame still need a fortnight of hardening off before they go into open ground. Move them out into full exposure by day and back under cover at night for a week, then leave them out entirely for another week. Sweet peas take this better than most plants because they were cold all winter, but the sudden move from a closed frame into wind is what breaks stems.
Plant out in early spring while the plants are still short, at six to nine inches (15 to 23 cm) apart along the base of the support. Plant at the depth they grew in the pot and water them in. Do not tease the root ball apart; keep the deep root column intact and drop it into a hole made with a trowel or a dibber deep enough to take the whole thing without bending it.
The ground should have been prepared in advance with plenty of compost or well rotted manure dug in, ideally the previous autumn. Unlike many annual climbers, sweet peas genuinely reward rich, moisture holding soil: they are hungry, thirsty and shallow rooted at the top even while the taproot runs deep. Poor dry ground gives short stems and an early finish.
Put slug protection down on the day you plant. Newly planted sweet peas are soft, low and exactly what slugs are looking for in a damp spring night, and a plant grazed off at the base is finished. This is the single most common way a carefully overwintered batch is lost in one night.